Learn Spanish (Latin America): Ustedes miran las flores y los árboles.

Both use a plural word for "the", but the article must match the gender of the noun. Flor (flower) is feminine, so it takes the feminine plural las. Árbol (tree) is masculine, so it takes the masculine plural los. In Spanish, articles always change to match both the gender and the number of the noun they belong to.

Since flores doesn't end in "-a", how do I know it's feminine and needs las?

The singular form is la flor. Words that end in consonants like "-r" don't follow the simple "-o is masculine, -a is feminine" rule. La flor is simply a feminine word you have to memorize! When it becomes plural, it keeps its feminine gender, becoming las flores.

Where is the word for "at" in this sentence?

You don't need one! The Spanish verb mirar means "to look at" or "to watch" all by itself. So, ustedes miran directly translates to "you look at." Trying to add an extra word for "at" here would sound incorrect to a native speaker.

Why does árboles have an accent mark?